Generally, glass fiber forming is performed in the following manner. A plurality of orifices are formed on a bottom of a bushing which accommodates molten glass. Glass fibers are flown out through the orifices, and a plurality of the glass fibers are bundled and wound up. In this process, in order to obtain uniform glass fibers, the bushing needs to be heated uniformly. The heating is performed by applying an electric current to the bushing.
The bushing temperature control has hitherto been performed in such a manner that a thermoelectromotive force of a thermocouple arranged in a center of the bushing is converted into a value of temperature and a predetermined amount of current is applied to the bushing so as to keep the temperature constant.
However, both ends of the bushing tend to have high current densities and to have temperatures higher than the temperature in the center. In such a case, because of the non-uniform temperature distribution of the bushing, the Tex of the glass fibers flowing out from the bushing becomes non-uniform, and quality defects are caused.
In order to prevent such a non-uniform temperature distribution of the bushing, the following techniques of controlling the bushing temperature have been proposed.
The Japanese Patent Laid-Open (Tokukai) No. S-61-228512 publication describes a three-terminal controller for a bushing, which includes a power transformer for supplying a current to the whole of the bushing. A region to which the current is supplied is divided into two regions. The three-terminal controller allows a current applied to each divided region to be regulated by a current bypass circuit using a thyristor. This three-terminal controller for the bushing is designed to prevent the non-uniform temperature distribution of the bushing by regulating amounts of the currents applied to the divided regions.
The Japanese Patent Laid-Open (Tokuhyou) No. H-2-500835 publication describes a bushing temperature controller which includes first, second, and third power transformers. The first transformer supplies a current to the whole of the bushing. A region to which the first transformer supplies the current is divided into three regions. The second and third power transformers separately supply currents to the regions in both ends. This bushing temperature controller is designed to prevent the non-uniform temperature distribution of the bushing by regulating amounts of the currents applied to the regions in the both ends.
Furthermore, the Japanese Patent Laid-Open (Tokukai) No. H-11-327661 publication describes a temperature controller which includes a transformer and shunt circuits. The transformer supplies a current to the whole of the bushing. A region to which the transformer supplies the current is divided into four regions. Each of the shunt circuits bypasses the current flowing through each region. This temperature controller is designed to facilitate segmentation of the region at low costs by forming the shunt circuits with wires and switches.